Air conditioning systems for motor vehicles typically include a compressor, a condenser, an evaporator, and an accumulator arranged as a refrigerant circuit. Proper lubrication of the compressor is usually dependent on oil circulating with the refrigerant through the system, and therefore the compressor is susceptible to damage if the mass flow rate of the refrigerant diminishes past a certain critical level. This can occur, for example, when refrigerant volume is lost through leakage or otherwise. It is therefore desirable to detect the approach of such a condition and alert the operator of the vehicle so that some action can be taken before the compressor is damaged. In the event that no preventative maintenance is performed, it is desirable to shut down the compressor automatically.
Various low refrigerant charge detecting devices have been proposed. U.S. Pat. No. 4,545,212 to Noda, for example, discloses a superheat detector including a semiconductor pressure sensor and a semiconductor temperature sensor. An operational controller converts an output signal from the semiconductor pressure sensor to a value corresponding to a saturation temperature of the refrigerant, and produces an output electrical signal corresponding to super-heat condition of the refrigerant by comparison between the saturation temperature and the output from the semiconductor temperature sensor. Similarly, U.S. Pat. No. 4,677,830 to Sumikawa et al. discloses a system wherein the temperature and pressure of refrigerant at the outlet of the evaporator are detected, respectively, by a temperature sensor and a pressure sensor, and the flow rate of refrigerant within the refrigerating cycle is controlled by controlling the opening of an expansion valve. The pressure of refrigerant detected by the pressure sensor is converted into a corresponding saturation temperature, and determining means determines whether the refrigerant quantity is insufficient by comparing the difference between the refrigerant temperature and the corresponding saturation temperature of the refrigerant with a predetermined reference value. When the refrigerant quantity is determined to be insufficient, a valve control means causes disengagement of the electromagnetic clutch of the compressor.